For a variety of computerized applications, it is desirable to measure the surface geometry of a three-dimensional (3-D) object by acquiring a multitude of data points that may be used for 3-D computation purposes or for 3-D display on a computer screen. Applications for object digitizing include anatomical/medical imaging and analysis; prosthesis design and manufacturing; animation/imaging applications; computer aided design (CAD) modeling; prototype design and develop; model, sculpture, and part duplication; stereolithography and laser-sintering data acquisition; as well as quality assurance and analysis.
Although a variety of methods and systems exist for acquiring surface data for these applications, no single method or system provides an economical and general solution to the object digitizing problem. One method to digitize objects in 3-D uses radiometric imaging (e.g., CAT scan nuclear magnetic resonance, or ultra-sound technologies). For medical diagnostic purposes these techniques are attractive, because they are fully-automated, fast, and may be safely applied to living subjects. For other purposes, however, these methods and the systems employing them are woefully inadequate. This is primarily due to their relatively high expense and lack of object image accuracy and resolution.
Another known type of system includes robotic tactile 3-D digitizers (i.e., machines that physically touch the surface of an object). While these machines provide high degrees of accuracy and resolution, they unfortunately are relatively slow and clumsy. This is because these types of machines must take precautions to prevent any disruptive or invasive contact between the digitizer and the object. For complex objects, obstruction avoidance makes tactile 3-D digitizers highly sophisticated and makes it necessary for them to possess robust sensing systems. Robotic tactile 3-D digitizers, therefore, are generally very expensive, slow, and limited to applications with simple, well-defined environments where the geometry of the object is partially known by the system prior to digitizing.
In an effort to overcome the expense and other limitations of the above methods and systems, active digitizing techniques have been developed. These techniques project energy to the object and measure the properties of any reflected energy image to determine the deflection location at the surface. These methods are attractive because they are non-destructive, relatively accurate and sufficiently resolute for many purposes. Digitizing speed may vary significantly depending on how much of the surface the digitizer illuminates and how many degrees of mechanical freedom exist between the object and the energy source. A degree of freedom simply describes the mechanical ability to move a sensor relative to the object. For example, a system in which an object may rotate about a central axis and the sensor translates up and down is said to have two degrees of freedom (DOFs).
Systems which employ two translational DOFs (i.e., vertical and horizontal translations) between the object and the ranging device may sample at rates up to 100,000 points per second. Systems utilizing three or more DOFs typically obtain as few as 10,000 points per hour. This slower sampling rate is directly attributable to the fact that positioning with 3 and 4 DOFs requires more time.
Probably the most popular type of 3-D digitizers use active triangulation of light energy. These devices are generally the most attractive approach available for accurate, high-resolution, non-destructive and fast surface measurement. Improved sampling speeds are typically obtained by using patterned light projection. Sophisticated lighting patterns require specialized two-dimensional sensors such as charged coupled device (CCD) cameras and extensive image processing facilities. While these digitizers may be fast, the electronic hardware they use is generally expensive, complicated, unreliable, and limited in application. The simplest form of pattern lighting is produced by projecting a single plane of light to the object. The linear surface contour illuminated by a plane of light may be quickly digitized with a CCD detector producing a large number of points for each line. The number of points typically equals the resolution of the CCD array detector. This technique is very popular and is commercially available from a variety of vendors.
Unfortunately, while this system provides rapid sampling rates, light-plane projection limits the application of these systems to simple convex surfaces that require only two translational degrees of mechanical freedom between the object and the light projector. If more degrees of freedom are used, this type of system will produce large amounts of redundant surface sampling which must be identified and removed when processing the subsequent large amount of acquired data.
It is important to recognize that these methods utilize imaging optics in which the reflected light from the object surface is collected and pseudo-focused onto a linear or planar array detector. This off-axis photo direction sensor method of active triangulation is basically a wide angle camera system for imaging the illuminated surface on to a detector. Disadvantages to this approach include variable accuracy and resolution, the need for calibration procedures, and the inability to employ scanning procedures that use more than two translational degrees of freedom without introducing sample redundancy. Variable accuracy and resolution of the off-axis photo direction sensors are a result of (1) a fuzzy image due to the inability to focus on the object; (2) the optics of this method produce large non-linear image distortions; (3) as the triangulation angle decreases, the accuracy and the resolution of the image also decreases; and (4) the accuracy and resolution of this technique is also limited by the accuracy and resolution of the detector.
As a result of the above, it is clear that there is the need for a 3-D object digitizing method and system that is relatively inexpensive and useful to measure ranges between a known point and points on the surface of an object for a wide variety of complex shaped objects.
There is the need for a 3-D object digitizing method and system that accurately provides geometrically and topologically complete high-resolution surface data.
There is a need for a method and system for 3-D object digitizing that eliminates measurement redundancy and has the ability to use a large variety of scanning procedures using different combinations of 2, 3 and 4 degrees of freedom.
Furthermore, in many situations, it is inappropriate to mathematically model a surface. When a physical model exists or can be constructed, it can be much more efficient to simply measure the surface and avoid any mathematical representation. Thus, there is a need for a 3-D object digitizing system that avoids the need to mathematically model a surface to provide accurate, high resolution 3-D object data.
Furthermore, there is the need for a 3-D object digitizing method and system that provides the ability of advanced interfacing with a wide variety of computer processing devices.